North Korea is reportedly preparing to fuel a long-range rocket ahead of a planned launch that Japan, South Korea and the United States say is a ballistic missile test.

South Korea's Yonhap news agency is reporting there has been increased activity near the fuel storage areas at the launch site in the north-west of North Korea.

The three-stage rocket was installed on the launch pad earlier this week, with sources in South Korea saying that fuelling the rocket could begin within 24 hours.

Pyongyang says it could launch the rocket as early as Monday.

North Korea says the rocket is simply taking a satellite into space, but the international community sees it as a test of ballistic missile technology, which is banned under UN Security Council resolutions.